Some speculate that the universe is so large we couldn’t possibly be alone. But there’s a companion thought experiment that is more interesting:

In a universe 14 billion years old, surely there is an advanced civilization somewhere older than ours (5000-10,000 years, depending how we define civilization) who has mastered interstellar communication or travel. So why haven’t they been in contact?

The Eerie Silence posits a few reasons why, terming them “filters” that tended to be dark or ominous; e.g., civilizations self-annihilate themselves through war or catastrophic scientific experiments.

The people at waitbutwhy.com have a more positive take, placing the filter at the front end instead of the back, which would mean civilizations are just rarer (much) than thought. Either the filter is behind us or we’re back to the pessimism found in Eerie.

In other words, “We’re rare, we’re first, or we’re f-cked.”

RARE: The creation of complex life takes, well, eons. While there may be many Earth-like planets, the particular conditions on Earth…are exceptionally friendly to life.

After prokaryotes came into being, they remained that way for almost two billion years before making the evolutionary jump to being complex and having a nucleus. If this is The Great Filter, it would mean the universe is teeming with simple prokaryote cells and almost nothing beyond that.

FIRST: We’ll be the first to self-annihilate or contact (or wipe out!) late-comers.

(I)t could be that the first chunk of the universe’s existence was full of cataclysmic events like gamma-ray bursts that would incinerate everything nearby from time to time and prevent any life from developing past a certain stage. Now, perhaps, we’re in the midst of an astrobiological phase transition and this is the first time any life has been able to evolve for this long, uninterrupted.

F#@&ED: If we’re neither rare nor early…

This would suggest that life regularly evolves to where we are, but that something prevents life from going much further and reaching high intelligence in almost all cases—and we’re unlikely to be an exception.

One possible future Great Filter is a regularly-occurring cataclysmic natural event, like the above-mentioned gamma-ray bursts, except they’re unfortunately not done yet and it’s just a matter of time before all life on Earth is suddenly wiped out by one. Another candidate is the possible inevitability that nearly all intelligent civilizations end up destroying themselves once a certain level of technology is reached.

This is why Oxford University philosopher Nick Bostrom says that “no news is good news.” The discovery of even simple life on Mars would be devastating, because it would cut out a number of potential Great Filters behind us. And if we were to find fossilized complex life on Mars, Bostrom says “it would be by far the worst news ever printed on a newspaper cover,” because it would mean The Great Filter is almost definitely ahead of us—ultimately dooming the species. Bostrom believes that when it comes to The Fermi Paradox, “the silence of the night sky is golden.”